At the present time, attempts are being made to recover heat energy from fuel immediately before combustion, in order to improve heat energy recovery from the different air, oil and fuel circuits. The calorific value of the fuel at injectors is high and forms an energy reserve that can be used in other circuits.
The maximum fuel temperature limit is usually fixed to satisfy safety requirements, and has to be modified in order to optimise recovery of heat from fuel. Therefore, heat exchangers have to be controlled and the temperature of the fuel in the tank has to be monitored before it is injected into the combustion chamber.
When an oil/fuel exchanger is located before the combustion chamber, heat exchanges between the engine lubrication oil circuit and the fuel regulation circuit are used to recover energy from the fuel if the temperature exceeds the maximum temperature while keeping the fuel at its temperature in the combustion chamber. This energy recovery helps to adjust the oil temperature to optimise energy exchanges with the fuel.
On the other hand, this operation is possible if the calorific value of oil is sufficient to absorb a certain quantity of heat. Existing architectures then comprise an air/oil exchanger to control temperatures in the oil circuit. This control facilitates heat exchanges with the fuel by controlling the oil temperature. Consequently, exchanges between the lubrication oil circuit and the fuel regulation circuit will be facilitated by minimising heat exchanges between the oil circuit and the air circuit while respecting maximum allowable temperatures in the oil and fuel circuit.
A device for performing this function to control the thermal power evacuated from the oil circuit to the air circuit through an oil/air exchanger, also called ACOC, and therefore to control the thermal power evacuated to the fuel circuit through the main oil/fuel exchanger called the MHX, is to include a temperature control valve. This type of valve is usually called an FTC (Fuel Temperature Control) valve.
At the present time, one solution is to use such a valve comprising a first channel without an exchanger that can be regulated in flow and a second channel with an oil/air exchanger to modulate the temperature of the incoming flow into this second channel. The flow in each channel can be modulated by a flow regulation device such as a slide.
However, the conventional configuration of the FTC valve is complex because it is based on a principle of modulating the opening sections of each channel while keeping a constant total flow and controlling the flow distribution by a computer. The slide includes two inlets that must be adjusted to the valve inlets.
FIG. 1 shows a currently used FTC valve in which a lubrication oil circuit is supplied by a lubrication element HP/LP LUB that comprises at least one high pressure circuit and one low pressure circuit. In general, the lubrication element comprises a set of pumps, for example geared pumps, filters and a distribution network comprising connections to equipment in an engine architecture, for example for an aircraft.
In the example shown in FIG. 1, the lubrication element HP/LP LUB comprises a low pressure outlet LP1 that transfers an oil flow in an exchanger E1 that usually maintains a minimum oil temperature. The purpose of this exchanger is to heat the oil flow passing through it. The oil flow heated to at least a minimum temperature is then transferred to an FTC type valve 2. A first flow fraction passes through a first channel comprising an ACOC type exchanger E2, in other words an air/oil exchanger and a first flow regulator 4. A second flow fraction passes through a second channel comprising a second flow regulator 3.
The valve 2 comprises two independent oil circuits:
a first control oil circuit 5, 6 comprising high pressure HP1 and low pressure LP2 oil exchanges; this oil circuit comprises two sub-networks independent of each other; one high pressure sub-network and one low pressure sub-network.
a second heat transfer oil circuit 7, the heat transfer oil circuit being derived from recoveries used for air/oil heat exchanges in exchangers E1, E2. This circuit is usually a low pressure circuit LP1.
The oil flow at the outlet 8 from the valve is then transferred to an oil/fuel exchanger E3 to guarantee a given required fuel temperature within a fuel tank R1.
The lubrication element HP/LP LUB controls the control oil circuit particularly through the inlets/outlets to the FTC type valve 2 or the tank R1.
The assembly 1 forms a calorific value exchanger unit to recover energy through calorific value, particularly by controlling the different heat exchanges in the air/oil and oil/fuel exchangers.
FIG. 2 shows an example of existing FTC valve solutions to guarantee a constant outlet flow 24 from the valve 2 starting from an inlet flow 26 distributed at the two inlets 22, 23 to the valve in a first flow 25 in which there is no heat transfer and a second flow 27 entering an oil/air exchanger E2.
One particular feature of this solution is that it requires an expensive current controlled servo-valve and that it outputs a control pressure to a slide 21 to control its position.
The slide 21 comprises two inlets 23, 22 and one outlet 28 that transfers the outlet flow 24 to an oil/fuel exchanger not shown in this figure.
This solution also requires an LVDT type position sensor for slaving the slide position. The position of the slide controls the fraction of the flow 25 and the flow 27 that are mixed in the outlet flow 24. The distribution of the flows 26 and 27 controls the oil flow temperature towards the oil/fuel exchanger.
It is desirable that the slide control pressure be proportional to the required opening section in one of the two channels to distribute the flow fraction that will be or will not be heated.
This valve solution has major disadvantages due to the integration of complex and expensive elements such as the servo-valve, the matched slide and the position sensor.
The servo-valve used as the section regulation device for controlling the flow and the matched slide are also elements considered to be sensitive to pollution and have poor resistance to fire.
The principle of controlling heat exchanges by distribution of flows requires the implementation of a proportional control servo-valve and a position sensor for looping in position. This requires a complex, sensitive and expensive architecture, while oil temperature control requirements at the valve outlet can tolerate some inertia in its slaving reactions.